Electrolytic vat



May 11, 1934. 7 v OBIEDQFF 1,957,283 I ELECTROLYTIC VAT Filed Feb. 15, 1932 51g i Patented May I, 1934 UNITED STATES- ELECTROLYTIC VAT Vladimir Obiedofl", Paris, France, assignor to Societe de Produits, Chimiques des Terres Bares, Paris, France, a corporation of France.

Application February 15, 1932, Serial No. 593,182 In Germany February 17, 1931 6 Claims. (Cl. 20419) It has long been known that the presence at iron in the electrolyte is highly prejudicial to the electrolysis of magnesium chloride, this being due to the successive oxidations andreductions of ferrous and ferric ions, and to the loss of current absorbed by such oxidations and reductions. Furthermore, theiron which is deposited with the magnesium constitutes a serious impurity'in this latter.

It is therefore most important to avoid all causes of introduction of iron salts into the bath.

It has however been found very difiicult,in vats employed for the production of magnesium by the electrolysis of its chloride, in spite of all precautions, to prevent the cathode metal above the bath especially after long use, from being attacked by the chlorine evolved. Even when the cathode consists of cast iron, which is relatively little affected, there is a formation of chlorides 20 of iron, which material is liable to get into the bath, and thereby cause trouble, as indicated above.

Furthermore, it is known that during theelectrolysis of magnesium, the metal of the electrode becomes, to a substantial extent, dissolved in the magnesium which is formed, and this may cause serious trouble, especially when the electrode consists of iron, as this metal is an impurity which greatly reduces the quality of the magnesium obtained. 7

The present invention relates to improvements in electrolytic vats, by which all such drawbacks will be obviated. According to my invention, I proceed in the first place by means of a systematic cooling of the cathode during the electrolysis, since the quantity of metal introduced into the magnesium increases with the temperature of the electrode.

When the cathode is considerably cooled, the

magnesium is deposited thereon in the solid state,

instead of collecting in liquid globules; the cathode can thus be covered with a protecting layer of magnesium, which prevents all contact between the bath and the metal of the cathode which may introduce impurities into the latter, and in this manner the magnesium is deposited upon magnesiumand not upon iron, this being considered as the limiting case. An equilibrium.

can thus be established for a given temperature and for a given thickness of magnesium deposited, at which the interior metal surrounding the cathode will be solid while 'the exterior metal in contact with the electrolyte is still liquid.

The temperature employed is preferably the lowest temperature compatible with the regular operation of the electrolysis, that is, the'temperature below which there would take place a continuous solidification of the magnesium,- or even of the electrolyte, around the cathode.

According to another precaution by which the introduction of iron will be obviated, the electrode I projects into the bath of chloride at beneath the surface of this bath, and thus all contact between apart of the electrode and the atmosphere in the upper part of the cell, which contains a large amount of chlorine is avoided.

The electrode is preferably inserted at the bottom or at the side of the vat; and it is to be remarked that by virtue of this arrangement the electrode is very readily cooled.

The following description, with reference to the accompanying drawing which is given by way of example, relates to various embodiments of the invention.

Figure 1 is a cross section of the bottom part of an electrolytic vat, showing the means employed for cooling the cathode.

Figure 2 is a cross section of a like vat, in which the cathode is inserted at the bottom.

Figure 3 is a cross section of a vat, with the cathode inserted at the side.

Referring to Figure 1, it is observed that the cooling of the cathode may be effected, for long and relatively narrow vats, by causing a current of air to travel in a vault 1, in which is situated an extension of the electrode 3, namely a metal part 2 outside the vat. The heat conductivity of the metal assures a suflicient reduction of temperature for the part of the electrode located in the vat, and in contact with the electrolyte.

It is further possible to use-a hollow electrode adapted for the circulation of air or even of water.

According to the invention, it is preferable continually to observe the temperature of the electrode, and to maintain it constant, at the optimum temperature. For this purpose a thermoelectric couple is used, and its outer sheath may consist of the cathode itself, or may be secured to this latter,

It will be seen from the above, that the surface of the cathode is cooled by conduction oi! heat therefrom into the cooler vertical projection 2 (in Figs. 1 and 2) or the cooler horizontal projection 5-(shown in Fig. 3) whereby the surface of the cathode is kept covered by a thin coating of solid magnesium, while the bath of molten MgCh is kept at not much above its meltingpoint. The

bulk of the magnesium formed, is in the molten state, but at a temperature only slightly above its melting point.

It is also feasible to connect with the cathode the metallic accessory parts extending into the vat, such as theiron tubes 7 (Figure 1) employed for the discharge of the metal which is formed; these tubes are cooled by conduction of heat, as they are in contact with the cathode to which they are secured, and thus they will not introduce iron into the magnesium which is formed. This method of proceeding also has the advantage of giving to such metallic accessory parts a polarity which tends to prevent the metals of which they are composed from being dissolved by the formation of chlorides.

To prevent the cathode from attack by chlorine, it is possible, as shown in Figure 2, to introduce the cathode 3 at the bottom, so that it will extend vertically upwards. It is thus situated at the center of the vat, between two longitudinal partitions 4 which form a chamber in which the metal may collect. By this arrangement, the lower portion of the cathode may extend into a vault 1 adapted for the circulation of air, thus facilitating the cooling of theca-thode.

In the arrangement shown in Figure 3, the cathode 6 enters at the side of the vat, and as before, the outer portion of the cathode may extend into a vault arranged for the purpose of cooling the said cathode.

The metal which is formed will thus flow upwards along the cathode, and will collect above it in a liquid layer while the cathode is entirely immersed in the bath and out of all contact with the atmosphere of the vat.

I claim:

1. Process for obviating the contamination of magnesium by iron from the cathode, during the electrolysis of magnesium chloride, which comprises maintaining the cathode at a temperature which is not considerably above the melting point of magnesium.

2. A process as claimed in claim 1, in which the cooling of the cathode is effected by a current of a cooling fluid.

3. A process as claimed in claim 1, which consists in observing the temperature of the cathode by means of a. thermo-electric couple, and in maintaining the temperature of the cathode surface at not considerably above the melting point of magnesium.

4. A process as claimed in claim 1, in which cooling of the cathode is effected by contact of a portion of the cathode outside the electrolyte with cooling air.

5. A process as claimed in claim 1, which consists in cooling not only the cathode, but also the accessory metallic parts, by providing a heat-conducting connection between such accessory parts and the cathode.

6. A process as claimed in claim 1, which consists in placing the cathode or cathodes entirely below the surface of the bath of molten magnesium chloride, thus avoiding all contact between the cathodes and the atmosphere in the cell.

VLADIMIR OBIEDOFF. 

